In a radio communication system, a transmitter may usually adopt multiple antennae to acquire a higher transmission rate. Multiple antennae may increase a signal to noise ratio and support a larger number of spatial multiplexing layers. Compared with an open-loop Multi-input Multi-output (MIMO) technology in which a transmitter does not use Channel State Information (CSI), a MIMO technology (closed-loop MIMO precoding) using CSI information can achieve higher capacity, and is a widely used transmission technology of a current mainstream 4th-Generation (4G) standard. A core concept of the closed-loop MIMO precoding technology is that a receiver feeds back channel information to a transmitter and the transmitter adopts some transmitting precoding technologies according to the obtained channel information to greatly improve transmission performance. For single-user MIMO, a precoding vector relatively matched with channel characteristic vector information can be directly adopted for transmitting precoding. For multi-user MIMO, relatively accurate channel information is also needed for performing interference elimination. Therefore, channel information acquisition at a transmitter is very important.
In some 4G technology standard specifications such as Long Term Evolution/Long Term Evolution-Advanced (LTE/LTE-A) 802.16m, an ordinary flow for acquiring downlink channel information of a Frequency Division Duplexing (FDD) system is as follows.
S1: a transmitter (evolved Node B) transmits downlink Channel State Information-Reference Signals (CSI-RSs) to a receiver. Generally speaking, each antenna transmits one CSI-RS. Positions of the CSI-RSs transmitted by different antennae are staggered on time and frequency domains or on a code domain, which can keep orthogonality and avoid mutual interference. Each antenna corresponds to one CSI-RS port. The CSI-RSs are used for measuring channel information. In LTE-A, CSI-RS transmitting of maximally 8 antenna ports on an evolved Node B side is supported. The evolved Node B also transmits Radio Resource Control (RRC) signaling for configuring related position information and transmitting period information of the CSI-RSs to a terminal. Contents of the CSI-RSs transmitted by the evolved Node B side are determined by some predetermined rules, so that the terminal can accurately learn about the contents of the CSI-RSs transmitted by each port on the evolved Node B side at each time-frequency position.
S2: the terminal receives configuration information of the CSI-RSs from the evolved Node B side, and performs CSI-RS reception and detection at a transmitting time-frequency resource position of each pilot port notified by signaling. On a terminal side, each receiving antenna obtains the received CSI-RSs. Since the terminal and the evolved Node B have reached an agreement on the contents of the CSI-RSs transmitted at each time-frequency resource position of each transmitting port, the terminal can accurately learn about the downlink CSI-RSs. Further, the terminal can further perform downlink channel estimation according to the received CSI-RSs to obtain downlink channel response information between ports of the receiving antennae of the terminal side and ports of the transmitting antennae of the evolved Node B side. During downlink channel estimation, it is needed to consider influence of doping of noise and interference during practical pilot signal reception. In view of the above, channel estimation may be performed by adopting algorithms of Least Square (LS), Minimum Mean Square Error (MMSE), Interference Rejection Combining (IRC) and the like, so as to finally obtain a downlink channel matrix matched with the number of the transmitting ports at each time-frequency resource position.
S3: the terminal may estimate channel responses between receiving antennae and ports of multiple transmitting antennae according to the transmitting contents of the CSI-RSs of each pilot port and the received CSI-RSs on each receiving antenna. By virtue of the estimated channel responses, a channel matrix corresponding to each time-frequency resource position can be obtained, and optimal CSI can further be calculated according to the channel matrix. The CSI usually includes three types of information, i.e. Precoding Matrix Indicator/Channel Quality Indicator/Rank Indicator (PMI/CQI/RI) information. A precoding matrix, channel quality information and the number of transmission layers are fed back and recommended to the evolved Node B respectively. The terminal feeds back the calculated CQI/PMI/RI information to the evolved Node B through a control channel of an uplink physical layer or a data channel of the uplink physical layer. The evolved Node B determines the number of transmission layers, a coding and modulation scheme and transmitting precoding on the basis of the feedback information of the terminal.
It can be seen that a downlink CSI-RS plays a very important role in an acquisition process of CSI. The downlink CSI-RS usually influences accuracy of precoding information, channel quality information and the number of transmission layers, and further greatly influences transmission performance of MIMO.
In a 4G standard, all adopted downlink CSI-RS pilots are periodical CSI-RS pilots. On the time domain, considering that a change of a channel is not a sudden change but has a certain time-domain correlation and a correlation time is more than the duration of one subframe, i.e., 1 ms, not all subframes are required for transmitting the CSI-RS. All User Equipment (UE) can share CSI-RSs, so that the CSI-RSs are usually periodically transmitted. A concept of a periodical pilot is that an evolved Node B performs CSI-RS transmitting according to a certain periodical interval, and transmitting positions of the pilot may have different subframe position offsets. For example, a CSI-RS period and subframe offset in LTE-A are defined as follows.
Specifications regarding CSI-RS subframe configuration made in an LTE standard 36.211 are shown in the following table.
TABLE 1CSI-RS Subframe ConfigurationCSI-RS-SubframeCSI-RS PeriodicityCSI-RS subframe offsetConfigTCSI-RSΔCSI-RSICSI-RS(subframes)(subframes)0-45ICSI-RS 5-1410ICSI-RS-5 15-3420ICSI-RS-1535-7440ICSI-RS-35 75-15480ICSI-RS-75
In the table, ICSI-RS is a configuration parameter of a CSI-RS, and is valued within a range of 0-154. Different ICSI-RS values correspond to different periods and subframe offsets of the CSI-RS. FIG. 1 is a diagram of subframe positions for transmitting CSI-RS corresponding to a part of CSI-RS configuration examples, i.e. configurations corresponding to ICSI-RS=0, ICSI-RS=2 and ICSI-RS=5 respectively.
In terms of frequency-domain position, CSI-RSs exist in each Physical Resource Block (PRB) pair, and the same transmitting pattern is adopted for the same port in different PRB pairs. A pattern of a CSI-RS is shown in FIG. 2. A PRB pair may refer to specifications in an LTE 36.211 protocol, and a typical condition includes 12 frequency-domain subcarriers and 14 time-domain Orthogonal Frequency Division Multiplexing (OFDM) symbols.
In an LTE system, it is defined that 40 Resource Elements (REs) in one PRB pair can be used for CSI-RSs. The 40 REs are divided into 5 patterns, each pattern including 8 REs, as shown in FIG. 2. Each port for CSI-RS pilots averagely occupies one RE in one PRB pair, and all ports belonging to the same CSI-RS resource are required to be limited in a pattern # i shown in FIG. 2. At present, a set of CSI-RSs supports maximally 8 ports, so that there are 5 candidate positions when the number of ports is 8, there are 10 configurable positions when the number of ports is 4, and there are 20 configurations when the number of ports is 2.
In an LTE-A system according to a related technology, precoding processing is usually not allowed when an evolved Node B transmits a CSI-RS pilot. This is mainly because multiple UEs in a cell share CSI-RS pilots, and if precoding is performed on the CSI-RS pilots, precoding can be performed only according to a characteristic of a channel from the evolved Node B to one UE, which may cause influence on measurement of the other UEs and disable the other UEs to accurately measure physical channels between Nr receiving antennae and Nr transmitting antennae, while performing precoding according to characteristics of channels of the other UEs may disable this UE to accurately calculate and report its own CSI. Of course, in a massive antenna communication system under discussion, when the number of antennae is very large, in order to save pilot overhead and reduce feedback complexity as much as possible, in some scenarios with relatively weak multipath scattering, an evolved Node B may also transmit a periodical precoded CSI-RS pilot, and such precoded CSI-RS is usually called as a beam measurement pilot. FIG. 3 is a transmitting strategy for a periodical beam pilot. Energy of each beam pilot is concentrated in a certain direction to form a directional beam. A beam measurement pilot is transmitted at an interval of a certain time period, and a group of beam pilots is polled.
Besides the periodical CSI-RS pilot described above, a non-periodical CSI-RS pilot is proposed recently. A non-periodical CSI-RS is an instantly triggered pilot. The non-periodical CSI-RS pilot is usually dynamically transmitted for channel measurement of specific UE or UE group. The non-periodical CSI-RS may not be continuously transmitted, and exists in only one subframe. Therefore, non-periodical pilot triggering information is carried in a Physical Downlink Control Channel (PDCCH) or an Enhanced-PDCCH (ePDCCH).
After learning about a transmitting position of a non-periodical CSI-RS, a terminal can perform pilot detection at the corresponding position. A transmitting content of the non-periodical CSI-RS may be pre-acquired by the terminal, like a periodical CSI-RS, so that a downlink channel response between a receiving antenna of the terminal and a transmitting antenna of an evolved Node B can be estimated to acquire a channel matrix.
There are two typical non-periodical pilot transmitting manners. One non-periodical pilot transmitting manner is to perform transmission in a Physical Downlink Shared Channel (PDSCH) of a user required to adopt a non-periodical CSI-RS for measurement. The other non-periodical pilot transmitting manner is to allocate a non-periodical CSI-RS competition resource pool of all users in a cell and then configure resources to different users on the basis of the resource pool. As shown in FIG. 4, the non-periodical CSI-RS competition resource pool may be transmitting resource positions of a set of periodical CSI-RSs.
It is noticed that a non-periodical CSI-RS is usually oriented to a specific user or a specific user group rather than all users in a cell. In view of this, the non-periodical CSI-RS may can a precoding method, the number of ports can be effectively reduced, and a calculated amount of CSI feedback can be further reduced. Therefore, the non-periodical CSI-RS may be selected to be transmitted either in a precoded beam pilot form or in a non-precoded non-beam pilot form according to a requirement.
There may be certain limits in the flexibility of transmitting of a periodical CSI-RS and non-periodical CSI-RS, no matter beam pilots or non-beam pilots, in the related technology. Specifically, the limits in the flexibility are reflected by the following aspects.
Flexibility limit (1): the numbers of ports and transmitting patterns in PRB pairs are the same.                The number of CSI-RS pilot ports in a PRB pair a is the same as the number of CSI-RS pilot ports in a PRB pair b.        Multiple pilot ports in the PRB pair a and the PRB pair b have the same transmitting resource densities (RE densities) in the respective PRB pairs.        The multiple pilot ports in the PRB pair a and the PRB pair b have the same transmitting patterns in the respective PRB pairs (transmitting positions in the PRB pairs).        
Flexibility limit (2): transmitting densities of all CSI-RS pilot ports are the same.                A port i and a port j have the same pilot density in a PRB pair.        A port i and a port j have the same transmitting PRB pair number density.        A port i and a port j have the same transmitting PRB pair position.        A port i and a port j have the same time-domain density (periodical configuration) and have the same subframe offset.        
Flexibility limit (3): different subframes of periodical CSI-RS pilots have completely the same transmitting characteristics.                Numbers of CSI-RS pilot ports and Identities (IDs) in a subframe m and a subframe n are the same.        CSI-RS pilots in the subframe m and the subframe n occupy the same number of REs in PRB pairs (resource densities in the PRB pairs are the same).        Numbers and densities of PRB pairs occupied by the CSI-RS pilots in the subframe m and the subframe n are the same.        The CSI-RS pilots in the subframe m and the subframe n occupy the same PRB pairs positions.        
Service objects of CSI-RS pilots are diversified, channel fades corresponding to applied channels are also greatly different, interference environments and strength magnitudes are different, and meanwhile, different antenna topologies may exist on an evolved Node B side. As a result, adopting an inflexible CSI-RS design may implement design simplification, but cannot achieve high performance. The application performance of CSI-RS having a fixed transmitting parameter may not be high in a part of scenarios.